The invention relates to a method of optically recording and erasing information a disc-shaped recording element which comprises a substrate and a recording layer provided thereon which comprises a crystalline recording material. In this method the disc is rotated and exposed to the light of a recording spot which is moved diametrically over the disc and originates from a laser light beam which is focused on the recording layer and is modulated in accordance with the information to be recorded. As a result amorphous information bits are formed in the exposed places of the recording layer which bits can be read by means of weak laser light and which can be erased by means of an erasing spot originating from a laser light beam focused on the recording layer.
Such a method which is based on crystalline-amorphous phase changes is known, for example, from European Patent Application No. 0,135,370 and from the copending Netherlands Patent Application No. 8403817 in the name of the Applicants. In the known method a Te-Se alloy is used as a recording material which may comprise other elements. A disadvantage of this method is that the erasing time, i.e. the exposure time which is necessary to return the amorphous information area (bit) to the original crystalline phase, is long. For example the erasing time of a Te-Se-Sb alloy is, for example, 50 .mu.s.
During the recording of information, the reading thereof and erasing the information the recording element is rotated. The linear speed of the element is, for example, from 1 m/s to 15 m/s or more. Upon recording audio(sound)information according to the EFM (eight out of fourteen) modulation system, a linear disk (element) velocity is used of 1.4 m/s. In video information a linear speed of 10-15 m/s is used.
It is highly desirable to perform the erasing process at the same linear disk speed as the recording process, that is erasing in real time. Furthermore it is highly desirable to be able to erase the information during one rotation of the element. For practical applications this presents the very interesting possibility of recording new information directly over the existing information, in which after the existing information has been erased, the new information can be recorded immediately thereon at the same disk speed and in the same rotation run of the element. Such a direct overwrite cannot be realised in magneto-optical recording processes, in which at least one complete revolution of the recording element exists between erasing and re-recording.
In phase change recording with the above-mentioned erasing time of 50 .mu.s, an erasing process in real time with one revolution of the element also is not readily possible in practice. Even at a very low linear disc speed of 1.4 m/s a slot-shaped erasing spot having a minimum length of 70 .mu.m must already be used in that case. The erasing spot is the light spot of the laser light beam on the recording layer used in the erasing process. An erasing spot of 70 .mu.m meets with considerable practical disadvantages due to the required high laser power, as well as the positioning of the spot on the track. At higher disc speeds even an erasing spot having a length in the order of magnitude of mm is necessary, which cannot be performed in practice.
In the published Japanese Patent Application Kokai 60-177446 an optical recording medium is described the recording material of which satisfies the formula In.sub.1-x Sb.sub.x M.sub.y wherein M is selected from a group of 13 elements, mostly metals, x is 55-80% by weight and y=0-20% by weight. Upon recording information, the recording material is exposed to modulated laser light. In the exposed places, depending on the rate of cooling, either a semi-stable phase is formed which is termed a .pi.-phase, or a mixed phase of InSb and Sb. The .pi. phase can be transferred to the mixed phase by heating. So there is switched--i.e. recording and erasing process--between a mixed phase and a .pi. phase. This has for its disadvantage that upon converting the mixed phase two constituents are involved. As a result of this the number of switching times--recording and erasing--is restricted although, according to the Kokai, a repeated reproduction is possible. This is a stability problem which is not acceptable for the practical applications. Moreover, the speed of conversion of the mixed phase to the .pi. phase is limited because the two components InSb and Sb must be present in a suitable proportion before conversion takes place. In the said Kokai no information is given or details disclosed of the erasing process, particularly the speed of erasing and the quality of the erasing process--the magnitude of a possible rest signal although it is stated that the information can be removed by "scanning" with laser light.
An additional serious disadvantage is the low signal-to-noise ratio of the information bits, which does not enable video recording.